Theoretical Studies On Mechanisms Of The Typical Electrode Processes Using Cyclic Reciprocal Derivative Chronopotentiometry (CRDCP)

Chronopotentiometry is an important branch of electroanalytical methods. Although the chronopotentiometric experiments have already been carried out at the beginning of the20th century, this controlled-current method has been paid great attention and developed rapidly since the middle period of last century. Derivative chronopotentiometry (DCP), cyclic chronopotentiometry (CCP) and reciprocal derivative chronopotentiometry (RDCP) etc. are important improved chronopotentiometric methods. In1996, combining the advantages of these derivated chronopotentiometric methods, our team proposed cyclic reciprocal derivative chronopotentiometry (CRDCP). In CRDCP, dt/dE-E curves are recorded instead of conventional E-t responses. The S-shaped E-t curves are converted to peak-shaped curves, which provide well-defined electrochemical signatures and have higher sensitivity. Characteristic parameters obtained from the dt/dE-E peaks are proved to be related to the kinetic and thermodynamic properties. This novel method can be treated as cyclic voltammetry (CV) under controlled-current condition and provide insight view into the mechanisms of the charge transfer processes occurring at the electrode/solution interface. CRDCP possesses distinct superiority in both theoretical and experimental studies and has already been used in the study of electrode processes, reaction mechanisms as well as surface phenomena.In this dissertation, we aim to present the fundamental theory of CRDCP. Theoretical models for several typical electrode reactions are studied, especially the electrode processes coupled with adsorption. Six chapters are included:Ⅰ. IntroductionIn the first half of this chapter, we outline the brief history of chronopotentiometry and introduce the improved chronopotentiometric techniques. In the second half, we aim to present the theoretical fundamentals of cyclic reciprocal derivative chronopotentiometry (CRDCP). The research progresses of CRDCP are reviewed.Ⅱ. Study of reversible electrode processes with successive symmetrical and unsymmetrical programmed CRDCPIn this chapter, the theoretical study of reversible electrode processes using CRDCP with symmetrical and unsymmetrical programmed currents applied is presented. The main viewpoints are:(1) For symmetrical programmed current, the amplitudes of the successive currents are the same, i.e.I(t)=(-1)1+1Io, whereas the transition times of each current step, τi, have different values. The properties of the dt/dE-E curves corresponding to each current cycle differ a lot;(2) For unsymmetrical programmed current proposed in this work for the first time, the current applied successively reversed at each transition time steps to different amplitude. The use of this unique programmed current is advantageous versus symmetrical programmed current since the transition times obtained are equal to each other anticipatively. The dt/dE-E curves also coincide with each other for the successive cycles. The results obtained in CRDCP are analogous to those of cyclic voltammetry (CV);(3) The characteristic parameters obtained from the dt/dE-E curves in both cases are quantitatively related to the species concentrations adjacent to the electrode surface and afford simple diagnosis criteria to characterize the reversibility of electrode processes. Properties of reversible electrode processes have been further studied through the use of more than one current cycle.Ⅲ. Kinetic study of totally irreversible electrode processes using CRDCPIn this chapter, we present the theoretical expressions and mathematical analysis for totally irreversible electrode processes corresponding to the application of both symmetrical and unsymmetrical programmed currents. For two successive unsymmetrical programmed currents, the effect of the currents ratio b (b=|I2(t)/I1(t)|) on the dt/dE-E curves is discussed. The electrochemical behavior of totally irreversible electrode processes has been studied corresponding to the application of the unique unsymmetrical programmed Φm(Io) proposed in Chapter2. CRDCP characteristic parameters obtained for totally irreversible electrode processes are different from those of reversible electrode processes. A comparison between both mechanisms is given. Based on the mathematical derivation, alternative methods for kinetic measuring are described. It is prospected that CRDCP is convenient and applicable for studying the reversibility of the electrode processes in the form of CRDCP characteristic parameters.IV. Study on irreversible electrode reactions coupled with Henry adsorption using symmetrical CRDCPIn this chapter, CRDCP with successive symmetrical programmed currents applied has been extended to describe irreversible electrode reactions where Henry adsorption occurs at the electrode/solution interface. The numerical simulated dt/dE-E curves are sensitive to electrode reactions coupled with adsorption processes. Effects of parameters on dt/dE-E curves presented in this work involve adsorption constants (Kox and KRed) and kinetic parameters (kso and a). Characteristic parameters of dt/dE-E curves depend on the adsorption strength markedly and afford effective criteria to determine different cases. The characteristic behavior of dt/dE-E curves with successive currents applied gives distinct descriptions between two limiting cases-the "surface nature" and "diffusion character". Comparisons are made with reversible charge transfer reaction with Henry adsorption and uncomplicated irreversible electrode reaction.V. Theoretical study of the electrolysis sequence of adsorptive and diffusing electroactive reactants by CRDCPTheoretical modeling and analyzing of electrode process in the presence of adsorptive reactants are presented in this chapter using CRDCP with symmetrical programmed current applied. Electrode process proposed based on the assumption of the electrolysis sequence of diffusion and adsorbed electroactive species are described as "on first, off last" and denoted as Onfol model. The numerical simulated dt/dE-E waves obtained from the analytical potential-time expressions of the Onfol model present peaks during both "diffusion processes" and "adsorption processes". Influences of maximum surface excess, adsorbed monolayer thickness, adsorption coefficient and current steps on CRDCP characteristic parameters are discussed. CRDCP behavior allows us to distinguish the form of the electroactive species contributing to the system current and the electrolysis sequence can therefore be determined.VI. Numerical simulation study of reversible electrode reaction coupled with Langmuir adsorption using CRDCPIn this chapter, theoretical modeling and analyzing of reversible electrode reaction coupled with Langmuir adsorption are presented using CRDCP with symmetrical programmed current applied, in which the finite difference method is firstly used to solve the boundary value problem corresponding to the electrode processes. The algorithm described in this study is available to solve analogous difference equations with nonlinear boundary conditions. Distinct features of dt/dE-E curves for typical models of electrode processes are obtained. If the adsorption coefficients of the electroactive species are of different orders of magnitude, the predominant peak splitting of the dt/dE-E curves can be observed, corresponding to the adsorption peaks and diffusion peaks. The sequence of the diffusion and adsorption peaks can indicate the strong adsorption of either reactant or product.